Catalytic burner utilizing electrosprayed fuels

ABSTRACT

A catalytic burner wherein a liquid fuel is evaporated prior to catalytic combustion by employing an electrosprayer. The catalytic burner can be made into an electrical generator by the use of a thermal to electrical energy conversion (TEEC) module.

CROSS-REFERENCE TO OTHER APPLICATIONS

[0001] This application claims priority to U.S. Provisional ApplicationSerial No. 60/368,120 entitled “A Clean and Efficient Combustor based onCoupling of Electrosprays and Catalytic Grids” dated Mar. 27, 2002, thedisclosure of which is incorporated in its entirety herein by reference.

[0002] This invention was developed under a contract with the Departmentof the Army, DARPA, Contract No. DAAD19-01-1-0664. The government mayhave certain rights herein.

FIELD OF INVENTION

[0003] This invention relates generally to a catalytic burner, and morespecifically relates to a catalytic burner employing anelectrohydrodynamic liquid fuel dispersion system, generally referredherein as an electrosprayer.

BACKGROUND OF THE INVENTION

[0004] Many catalytic combustion applications use liquid fuels. Thesefuels, however, must be evaporated prior to combustion. One method ofevaporation uses a vaporizer wherein the liquid fuel is flowed through anozzle onto a hot surface, which causes the liquid fuel to evaporate.Generally, vaporizers require large energy expenditures to maintain thetemperature of the hot surface. In addition, when this method isemployed with heavy hydrocarbons, such as Diesel fuels and jetpropulsion fuels (e.g., JP-8, Jet A, etc.), equipment maintenance maybecome excessive. The nozzle, while not in contact with the hot surface,is in the presence of the hot surface therefore it gets quite hot. As aresult, the liquid fuel within the nozzle decomposes thermally to formvarious solid deposits and/or coke. These deposits and/or cokeeventually form blockages within the nozzle that foul or block thenozzle. Periodic maintenance is required to remove these blockages.

[0005] Yet another method of evaporation employs a spray atomizer.Generally, a spray atomizer is preferred for the evaporation of liquidfuel. A spray atomizer disperses the liquid fuel in fine droplets, in aprocess generally referred to as atomization, into a gas, such as air.The fine droplets increase the surface area of the liquid fuel, thus theinterface between the gas and the liquid fuel is increased.Consequently, the evaporation rate of the liquid fuel into the gas isincreased. These systems, however, create droplets of substantiallydifferent size and poorly disperse the droplets, which can effectoverall combustion performance.

[0006] Based on the foregoing, it is the object of the present inventionto overcome the problems and drawbacks of the prior art.

SUMMARY OF THE INVENTION

[0007] The invention is directed in one aspect to a method of catalyticcombustion wherein a liquid fuel, which is to be combusted, iselectrosprayed. In the method, a liquid fuel is electrosprayed into agas, which includes an oxidant for the fuel in the presence of acatalyst used in the catalytic combustion. The fuel evaporates withinthe gas forming a mixture that is catalytically combusted. Preferably,the liquid fuel is a hydrocarbon or an alcohol and the gas is air, whichcontains the oxidant oxygen.

[0008] In an apparatus, i.e. catalytic burner, of the present invention,an electrosprayer is positioned relatively upstream from a catalyst. Theterm upstream and conversely downstream, as used herein, is based on thenormal direction of travel of fluids, such as the fuel, through thecatalytic burner. In a preferred embodiment as further explained below,the catalyst is supported on an electrically conductive substrate whichacts as an electrode for the electrosprayer. The apparatus may also becombined with a thermal energy to electric energy module.

[0009] Electrospraying is accomplished by an electrosprayer, which is atype of atomizer. Typically in an electrosprayer, a fluid is forcedunder pressure through a nozzle creating a stream of fluid. A differencein electrical charge within the electrosprayer is created by a voltagedifferential between an electrode that imparts the electrical charge tothe stream of fluid and another electrode, which is positioned proximateto the nozzle. In one type of electrosprayer, the nozzle acts as anelectrode that imparts an electrical charge to the stream of fluid. Forbest operation, the voltage differential is quite large. To enhance theability of the fluid to accept an electrical charge, the electricalconductivity of the fluid may be increased by the addition of aconduction additive.

[0010] When an electrically charged fluid exits the nozzle, the fluidimmediately prior to discharge from the nozzle has a meniscus thatcauses the fluid to adopt upon exiting the nozzle a conical shape withan apex pointing downstream toward the charged electrode, which isgenerally at a lower voltage. As the fluid moves away from the nozzle, aligament develops that trails from the apex. The ligament is eventuallybroken into fine droplets by instabilities in the flow downstream of thenozzle. This results in the formation of substantially smaller dropletsthan those formed by other atomization techniques, such as sprayatomizers.

[0011] Electrospraying offers the advantages with respect to otheratomizing techniques of: producing a ligament that can be orders ofmagnitude smaller than the cross-section of the nozzle, which allows forthe generation of very small droplets from a large nozzle therebyreducing the potential for clogging of the nozzle; producing moreuniform droplets thereby making the rate of evaporation of the dropletsgenerally more uniform; and providing increased dispersion of droplets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a cross-sectional view of a catalytic burner.

[0013]FIG. 2 shows a cross-sectional view of the catalytic burner ofFIG. 1 in combination with a thermal energy to electrical energy (TEEC)module.

[0014]FIG. 3 is a graph illustrating the temperature across thecatalytic bed within the catalytic burner.

[0015]FIG. 4 is as graph of efficiency of a catalytic burner based on amass flow rate of 9.7 grams/hr. of n-dodecane.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] As shown in FIG. 1, a catalytic burner of the present invention,generally designated by the reference number 10, includes anelectrosprayer 12 having at least one nozzle 14. A catalyst 16 is placeddownstream of the at least one nozzle 14.

[0017] The catalyst 16 is positioned at the surface of at least aportion of a substrate 22. One or more substrates 22 may be securedwithin an annular ring 24 having a c-shaped cross-section into which theedges of the substrates 22 are placed thereby defining a catalytic bed26. The substrates 22 and annular ring 24 may be made from electricallyconductive material.

[0018] Still referring to FIG. 1, a housing 28, which may be made froman electrically conductive material, defines an opening 30. On a surface32 of the housing 28 opposed to the opening 30 is at least one nozzle 14oriented such that a fuel 18 exits the nozzle toward the opening. Thecatalytic bed 26 is placed across the opening 30. Insulators 34 areprovided between the catalytic bed 26 and the housing 28 to electricallyisolate the catalytic bed 26 from the at least one nozzle 14.

[0019] A direct current power source 36 has one lead 38 connected to theat least one nozzle 14, thereby defining the at least one nozzle as anelectrode. Another lead 40 from the power source 36 is connected to thecatalytic bed 26, making the catalytic bed, substrates 22 and annularring 24, another electrode.

[0020] A mixture flow conduit 42, defined by the housing 28, extendsbetween the surface 32 of the housing 28 and the catalyst 16. Thehousing 28 defines a port 44 exiting into the mixture flow conduit 42for introducing a gas 46 into the mixture flow conduit where it mixeswith the fuel 18 to create a mixture 47. The port 44, preferablypositioned to permit all the gas 46 to enter the mixture flow conduit 42upstream of the at least one nozzle 14, has a cross-section that may bechanged by a valve 48 to regulate the amount of gas flowing through theport.

[0021] A pump 50 connects to a fuel flow conduit 51, defined by at leastone surface 53, that is common to the least one nozzle 14. The pump 50pressurizes the fuel 18 so that it flows through the at least one nozzle14.

[0022] The catalytic burner 10 presented above can be used incombination with a conventional thermal energy to electric energy (TEEC)module to produce electricity, i.e. be an electrical generator. As shownin FIG. 2, the TEEC module 52 is placed downstream of the catalytic bed26 such that an exhaust gas 54, created by the catalytic combustion ofthe mixture 47 within the catalytic bed, impacts upon a heat surface 56of the module. Impacting of the exhaust gas 54 on the heat surface 56 ofthe module 52 assures maximum heat exchange between the exhaust gas andthe module. The heat surface 56 of the module 52 cooperates with thecatalytic bed 26 and the housing 28 to create an intermediate flowconduit 58 to an exhaust port 60, which is defined by the housing.

[0023] A cool surface 62 of the module 52 in cooperation with at leastone surface 64 defines a gas input flow conduit 66 that is in downstreamfluid communication with an exhaust flow conduit 68, which is defined byat least one surface 70, extending from the exhaust port 60. The gasinput flow conduit 66 is in downstream fluid communication with themixture flow conduit 42 via the port 44.

[0024] A pump 72, such as a fan, is provided at an entrance 73 into thegas input flow conduit 66 to provide a means to force the gas 46 throughthe gas input flow conduit. Increased gas 46 flow over the cool surface62 of the module 52 may increase the overall efficiency of the module.

[0025] A perforated plate 74 is positioned between the at least onenozzle 14 and the catalytic bed 26 within the mixture flow conduit 42.The perforated plate 74 acts as a mixer to assist in evaporation of thefuel 18 and to create a more uniform mixture 47.

[0026] The method of the present invention will now be explained withinthe context of the above-described apparatus. The fuel 18 is pressurizedby the pump 50 and forced though the at least one nozzle 14 towards thecatalytic bed 26. The direct current power source 36, which has one lead38 connected to the at least one nozzle 14, electrically charges thefuel 18 as it passes through the at least one nozzle.

[0027] As the fuel 18 exits the at least one nozzle 14, a ligament 76 iscreated as a result of the voltage difference between the fuel and thecatalytic bed 26. The ligament 76 extends from the at least one nozzle14 in the direction of the catalytic bed 26. Instability in the ligament76 causes the ligament to break into numerous droplets 78. At least aportion of the droplets 78 evaporate within the gas 46 and mix with thegas to create the mixture 47 that flows into the catalytic bed 26, whereit is catalytically combusted forming the exhaust gas 54.

[0028] In steady state operation, the catalytic combustion, which isexothermic (releases heat), will entrain the mixture 47 into thecatalytic bed 26, thereby giving the fuel 18 and the gas 46 within themixture flow conduit 42 a mass flow rate and a velocity. The gas 46 inthe mixture flow conduit 42 may be replenished by the natural draw ofgas through the port 44. The valve 48 may regulate the mass flow rate ofthe gas 46.

[0029] The mixture flow conduit 42 has a distance “d” between the atleast one nozzle 14 and the catalyst 16, generally assumed to be aninlet face 80 of the catalytic bed 26. The distance “d” and the velocityof the fuel 18 and gas 46 within the mixture flow conduit 42 define amaximum residence time for the fuel and gas within the mixture flowconduit. This in turn determines the amount of evaporation of the fuel18 and the degree of mixing of the evaporated fuel with the gas 46before the mixture 47 is brought into the catalytic bed 26.

[0030] The basic method may be modified when a TEEC module is employedwith the catalytic burner. As shown in FIG. 2, the gas 46 may be pumpeddown the gas inlet flow conduit 66. Pumping of the gas 46 gives the gasa mass flow greater than that caused by the natural draw of thecatalytic burner 10. The increased gas 46 flow over the cool surface 62of the module 52, however, may increase the efficiency of the module. Asthe quantity of gas 46 is above that needed by the catalytic burner 10,the excess gas 46 is discharged through the gas inlet flow channel 66,which is in downstream fluid communication with the exhaust gas flowconduit 68.

[0031] As those skilled in the art of catalytic combustion willappreciate, there are numerous combinations of fuels, oxidants, andcatalysts. It should be readily appreciated that the catalyst 16 and thecatalytic bed 26 and any mixture 47 could be engineered to permitpartial combustion or near total combustion within the catalytic bed 26.It should also be appreciated that in the case of partial combustion, aflame may develop downstream of the catalytic bed 26, depending uponwhether the exhaust gas 54 is within its flammability limits andignition can be achieved. It should also be appreciated that startup ofthe catalytic burner 10 may necessitate preheating of the catalyst 16.One possible method of preheating the catalyst 16 when the catalytic bed26 employs an electrically conductive substrate 22 is electricresistance heating of the substrate.

EXAMPLE

[0032] A catalytic burner, consistent with that depicted in FIG. 1, wasconstructed and operated in a manner consistent with the method. Thecatalytic burner was designed for use with a conventional TEEC modulecapable of delivering 20 Watts of electrical power. Based on a 20-Wattelectric power delivery requirement, it was determined that thecatalytic burner would have to have a thermal energy output on the orderof 100 Watts. Using a typical heating value of 42,500 J/gm forhydrocarbon fuels, a 100 Watt thermal energy output translated into acatalytic burner capable of combusting a hydrocarbon fuel at a fluidflow rate of 7-8 grams/hour.

[0033] The liquid hydrocarbon fuel JP-8 was selected. JP-8 was selectedbecause it is a readily available liquid hydrocarbon fuel that is wellsuited to electrospraying, once its electrical conductivity is enhancedby a suitable additive (e.g., an antistatic additive). Demonstratingsuccessful operation with JP-8 implies that operation with other fuelsis also possible, such as other n-dodecanes (a single component fuelhaving similar physical properties). Air was selected as the gas as itcontains the desired oxidant oxygen. The catalyst was 80% palladium and20% platinum deposited on an alumina washcoat. The substrate was aMICROLITH® substrate from Precision Combustion, Inc. of North Haven,Conn.

[0034] The number of nozzles and the arrangement of the nozzles werebased on the size of the droplet and dispersion of the dropletsnecessary to achieve the desired evaporation rate. As discussed above,the evaporation rate determines the size of the catalytic burner, as itsets the distance “d” needed to obtain the mixture.

[0035] The droplet size for any given fluid from a nozzle of anelectrosprayer is primarily a function of fluid flow rate therethrough.The greater the flow rate, the greater the droplet size. Thus, thenumber of nozzles required is a function of the droplet size and thetotal flow rate required. The greater the droplet size the greater theresidence time required, i.e., the longer the distance d, for thedroplet to evaporate and mix with the oxidant.

[0036] In order to meet the fuel flow rate required, a PEEK manifoldconnected multiple stainless steel capillaries, i.e., nozzles, arrangedin a hexagonal pattern. The capillaries were mounted in a hexagonpattern to maximize the number of capillaries in a given cross-sectionalarea and maintain each capillary at a fixed distance from one another,thereby minimizing interference between capillaries. The capillarieswere mounted through a flange capable of withstanding high temperatures,with the tips of each capillary sharpened and polished to eliminateburrs, which can affect the pattern of the electrical field createdwithin the electrosprayer. The flange was supported within a cylindricalPyrex housing, which was transparent, thereby permitting viewing of thespray pattern and electrically isolate the capillaries from thecatalytic bed. To simulate heat recuperation, which may be desired inthe complete system to increase the thermodynamic efficiency, the gaswas preheated to 500 degrees C. A voltage differential was createdbetween the catalytic bed and each capillary of several kV.

[0037] To characterize the performance of the catalyst, the surfacetemperature and the uniformity thereof of the catalytic bed and theexhaust gas composition were measured. The temperature on the surface ofthe catalytic bed was measured using a PV-320 Electrophysics infraredvideo camera with a germanium objective lens. The exhaust gascomposition was estimated by obtaining a series of samples, each beinganalyzed by a two-channel Micro-Gas Chromatograph from AgilentTechnologies.

[0038] A fuel equivalence ratio, i.e., the actual fuel/air ratio dividedby the stoichiometric fuel/air ratio, was selected based on therequirements of the assumed TEEC module. Variations in equivalence ratiomay allow for operation over a much broader temperature range.

[0039]FIG. 3 is a typical two-dimensional map of temperature of thecatalytic bed. This result corresponds to a fuel equivalence ratio of0.48 (a lean mixture) at a fuel flow rate of 9.8 grams/hour. Radialtemperature scans of the catalytic bed indicated a 5 percent, plus orminus, variation in temperature across an exit face of the catalyticbed. In this case the maximum temperature achieved was approximately1100 degrees K.

[0040] For a base case of operation with n-dodecane having a fuelequivalence ratio of 0.48 and a fuel flow rate of 9.7 grams/hr, gaschromatographic measurements of mole fractions of the main components,N₂, O₂, CO₂, and CO, in a dry gas sample of the exhaust gas yielded81.5%, 11.5%, 6.4%, and 0.11%, respectively. Light hydrocarbons such asCH₄, and C₂H₆, as well as H₂ were below detectable levels (550 ppm).This gives an estimated efficiency of combustion of 99%. As shown inFIG. 4, if CO₂/CO is used as a surrogate for efficiency, for a fuel flowrate of 9.7 grams/hr the maximum combustion efficiency for thiscatalytic burner occurs at a fuel equivalence ratio of 0.45.

[0041] It is particularly important to note that 100% combustion withina catalytic bed cannot be accomplished unless the catalytic bed isinfinitely long. Therefore, the language and terminology used hereinshould be interpreted within the context of catalytic combustionsystems.

[0042] The invention has been described in considerable detail based ona preferred embodiment. Therefore, the spirit and scope of the inventionshould not be limited to the description of the preferred versionscontained herein. For example, the pumps may be gravimetrically createdand other catalytic beds could be used. In addition, the electrodes donot have to be the components identified, nor does the mixture flowconduit have to be linear. These are but a few examples.

What is claimed is:
 1. A method of catalytic combustion comprising:electrospraying a fuel into a gas having an oxidant; evaporating theelectrosprayed fuel; mixing the evaporated fuel with the gas to create amixture; and catalytically combusting the mixture.
 2. The method ofclaim 1 wherein the fuel is a hydrocarbon.
 3. The method of claim 2wherein the gas is air.
 4. The method of claim 1 wherein in the step ofcatalytically combusting the mixture, the mixture is catalyticallycombusted within a catalytic bed having catalyst supported on a screensubstrate.
 5. The method of claim 4 wherein the screen substrate is madefrom a material that is electrically conductive.
 6. The method of claim1 wherein the step of catalytically combusting the mixture produces aflammable exhaust gas.
 7. The method of claim 1 wherein the step ofcatalytically combusting the mixture produces a nonflammable exhaustgas.
 8. A catalytic burner comprising; a housing defining a mixture flowconduit; an electrosprayer having at least one nozzle, the at least onenozzle positioned within the mixture flow conduit; and a catalystpositioned within the mixture flow conduit.
 9. The catalytic burner ofclaim 8 wherein the housing defines a port having an exit into themixture flow conduit and a valve is disposed within the port.
 10. Thecatalytic burner of claim 9 wherein the exit has a portion upstream ofthe at least one nozzle.
 11. The catalytic burner of claim 8 furtherincluding a substrate on which the catalyst is supported, the substratebeing made from an electrically conductive material, and theelectrosprayer has electrodes one of which is the substrate.
 12. Thecatalytic burner of claim 11 further including a c-shaped ring intowhich the support is placed.
 13. An electric power generation systemcomprising: a catalytic burner including a housing defining a mixtureflow conduit and a port having an exit into the mixture flow conduit, anelectrosprayer having at least one nozzle positioned within the mixtureflow conduit, and a catalyst positioned within the mixture flow conduit,and a thermal to electrical energy conversion module positioneddownstream of the catalyst.
 14. The electric power generation system ofclaim 13 further including a substrate on which the catalyst issupported, the substrate being made from an electrically conductivematerial, and the electrosprayer has electrodes one of which is thesubstrate.
 15. The electric power generation system of claim 13 furtherincluding a pump discharging into a gas inlet flow conduit wherein thegas inlet flow conduit is in downstream fluid communication with themixture flow conduit via the port.
 16. The electric power generationsystem of claim 15 wherein the mixture flow conduit is in downstreamfluid communication with the gas inlet flow conduit.